The present invention relates to a circuit for determining the speed of rotation of a wheel, the circuit having an inductive transmitter the output of which is connected to a pulse-shaping device whose output is connected to an evaluation device.
In motor vehicles, it may be necessary, for instance for an anti-lock system, to detect the speed of a wheel or the speed of rotation of a wheel. For this purpose, inductive transmitters are used in which a sinusoidal voltage, the frequency of which is proportional to the speed of rotation of the wheel, is induced by a periodically varying magnetic conductance, for instance a toothed rim arranged concentrically to the wheel and turning in synchronism with the wheel. The approximately sinusoidal voltage is further processed by a subsequent electronic system which produces a train of square pulses from the voltage. From the distance apart in time of given variables of these square pulses, for instance the ascending flanks, information can be obtained as to the speed of rotation of the wheel in question by means of a digital processing device. Since the voltage produced in inductive transmitters is proportional to the change of the magnetic field with time, the output voltage of an inductive transmitter is strongly dependent on the frequency. The higher the frequency, the greater the amplitude of the output signal.
Since the inductive transmitters must be arranged directly on the wheel, they are subject to outside influences to an increased extent. They are dirtied by road dirt and moisture, and strongly acted on mechanically by vibrations. The vibrations can, for instance, change the distance of the transmitter from the toothed rim or lead to the breaking of an electric cable. The evaluation of the voltage produced by the inductive transmitter is generally not effected directly on the wheel but rather in a central evaluation device to which the voltage is transmitted by means of electric cables. The dirtying can result in so-called leakage-current paths between the two wires of a cable. The dirt resistances impair the diagnosis of errors which, in some manner, evaluates the voltage drop over the ohmic internal resistance of the inductive transmitter. This voltage drop is produced when a direct current is impressed upon the transmitter. The voltage drop reduces the sensitivity of the pulse-shaping circuit considerably if no decoupling of the DC potential on the sensor from that on the pulse-shaping device is effected.